Common Stock and Share-Based Compensation - Restricted Stock Unit Activity (Details) - RSUs	6 Months Ended
Jun. 30, 2018 $ / shares shares
PSUs
Outstanding balance at beginning of period (shares)	631,234
Stock units issued (shares)	813,413
Shares issued upon vesting of stock units (shares)	(204,843)
Stock units canceled or forfeited (shares)	(165,948)
Outstanding balance at end of period (shares)	1,073,856
Weighted-average grant date fair value (USD per share) \| $ / shares	$ 11.58
Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
PSUs
Outstanding balance at beginning of period (shares)	545,405
Stock units issued (shares)	717,744
Shares issued upon vesting of stock units (shares)	(166,389)
Stock units canceled or forfeited (shares)	(164,033)
Outstanding balance at end of period (shares)	932,727
Weighted-average grant date fair value (USD per share) \| $ / shares	$ 11.52
Non-Employee Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
PSUs
Outstanding balance at beginning of period (shares)	85,829
Stock units issued (shares)	95,669
Shares issued upon vesting of stock units (shares)	(38,454)
Stock units canceled or forfeited (shares)	(1,915)
Outstanding balance at end of period (shares)	141,129
Weighted-average grant date fair value (USD per share) \| $ / shares	$ 11.98

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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Number of shares (or other type of equity) issued during the period as a result of any equity-based compensation plan other than an employee stock ownership plan (ESOP), net of any shares forfeited. Shares issued could result from the issuance of restricted stock, the exercise of stock options, stock issued under employee stock purchase plans, and/or other employee benefit plans.

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